1. Field of the Invention
The present invention relates to testing methods and apparatus. More particularly, the present invention relates to an apparatus for testing printing smoothness, ink absorbency and other characteristics of paper contributing to a mottled print appearance.
2. Background of the Invention
Numerous tests and testing devices have been devised by the prior art to prospectively evaluate the performance of a given lot of paper in a printing machine. Of these numerous devices and procedures, those provided by the Research Institute for the Printing and Allied Industries IGT have gained the widest acceptance by the printing industry. Such machines are generally characterized as IGT printability testers.
In essence, IGT printability testers are miniature printing machines having adjustment features for printing pressure and print speed. A paper sample (specimen) is mounted on a wheel sector for rotational nip contact with a print wheel. Rotational speed of the specimen sector is controllable as is the nip pressure between the specimen and the print wheel.
Ink film thickness on the print wheel is only indirectly controllable. The IGT printability tester print wheel is conveniently detachable from the tester unit for rotational mounting on a separate inking unit.
As a separate apparatus, the inking unit comprises two hard surfaced cylinders running on parallel axes bridged in driving nip contact by a resilient surface roller. One of the hard surface rollers axially reciprocates under rotation while the other is rotatively driven by a motor. The IGT printability tester print wheel is removed therefrom and rotatively secured to a swing bracket for rotative nip engagement with the resilient surface roller of the inking unit.
All mutually engaged rotating surface of the inking unit are of known surface area. When a known volume of ink is placed on the surface of one roller, the consequential film thickness may be calculated on the assumption that the ink has been uniformly distributed on all rotational surfaces. Such is the objective of the reciprocating roller to enhance the assumption of uniform distribution.
In operative test sequence, a printability tester print wheel is removed from the tester unit and positioned on the swing bracket of the inking unit to have a known ink film thickness applied to the surface thereof.
Having received such surface film, the print wheel is removed from the inking unit and returned to the printability test unit for specimen engagement.
In this test sequence, two extremely important parameters have been neglected by lack of control. First, there is no control over the nip pressure between the print wheel and the resilient ink transfer roller of the inking unit. This nip pressure influences the ink film thickness actually transferred to the print wheel surface.
More importantly, the time interval between print wheel coating on the IGT inking unit and specimen printing on the physically separated IGT printability tester is an uncontrolled variable due to the necessity to manually transfer the coated print wheel from the inking unit to the printability tester. Such time variations influence the tack of highly volatile solvent inks.
It is, therefore, an object of the present invention to provide a paper printability test unit wherein the nip pressure of the print wheel against an inking roller is precisely and repeatably regulated.
Another object of the present invention is to provide a paper printability testing unit wherein the inking and printing functions of a test sequence are integrated on a single frame.
Another object of the present invention is to provide a paper printability test unit having a precisely controlled time interval between inking and printing.